Melt crystallization purification of lactides

Abstract

Melt crystallization purification of lactide; and separation and purification of lactides existing in more than one isomeric form.

We claim:  
     
       1. A solvent-free method for the purification of a non-eutectic lactide mixture derived from the depolymerizaton of lactic acid oligomers comprising by weight a major amount of one lactide isomer and a minor amount of at least one other lactide isomer and acidic and other impurities comprising the steps: 
       (1) melting the lactide mixture;  
       (2) cooling the molten lactide mixture to the freeze point or slightly below the freeze point of the lactide mixture to effect partial crystallization of the mixture and formation of a solid phase having a higher content than the molten mixture of the major isomer and a lower content of the minor isomer(s) and acidic and other impurities;  
       (3) separating the solid phase from the liquid phase;  
       (4) forming an additional molten mixture by melting the separated solid phase; and  
       (5) repeating steps (2)-(4) with the additional molten mixture until a predetermined degree of putity is obtained in the separated solid phase.  
     
     
       2. A solvent-free method for the purification of a lactide mixture derived from the depolymerization of lactic acid oligomers comprising by weight a major amount of one lactide isomer and a minor amount of at least one other lactide isomer and acidic and other impurities comprising the steps: 
       (1) melting the lactide mixture;  
       (2) cooling the molten lactide mixture to the freeze point or slightly below the freeze point of the lactide mixture to effect partial crystallization of the mixture and formation of a solid phase having a higher contenty than the molten mixture of the major isomer and a lower content of the minor isomer(s) and acidic and other impurities;  
       (3) separating the solid phase from the liquid phase;  
       (4) forming a further liquid phase by gradually warming the separated solid phase to a temperature below its melting point;  
       (5) separating the solid phase form the further liquid phase; and  
       (6) repeating steps (2)-(4) until a predetermined degree of purity is obtained in the separated solid phase.  
     
     
       3. The method of claims  1  or  2  in which the lactide mixture is comprised of more than 80% by weight of the major isomer. 
     
     
       4. The method of claim  3  in which the major isomer is L lactide. 
     
     
       5. The method of claim  3  in which the major isomer is D lactide. 
     
     
       6. A solvent-free method for the recover of DL lactide from a non-eutectic lactide mixture derived from the depolymerization of lactic acid oligomers comprising between 20 and 80% by weight each of D and L lactide and a minor amount of acidic and other impurities comprising the steps: 
       (1) melting the lactide mixture;  
       (2) cooling the molten lactide mixture to the freeze point or slightly below the freeze point of the lactide mixture to effect partial crystallization of the mixture and formation of a solid phase comprising DL lactide and a lowe content of acidity and other impurities;  
       (3) separating the solid phase from the liquid phase;  
       (4) forming an additional molten mixture by melting the separated solid phase; and  
       (5) repeating steps (2)-(4) with the addtional molten mixture until a predetermined degree of purity is obtained in the separated solid phase.  
     
     
       7. The method of claims  1 ,  2  or  6  in which the acidity of the separated solid phase is reduced to below 10 meq/kg. 
     
     
       8. The method of claim  1  or  2  carried out in a falling film crystallizer. 
     
     
       9. The method of claim  2  or  6  carried out in a static crystallizer. 
     
     
       10. The method of claim  1  carried out in a zone melt purifier. 
     
     
       11. A process for the melt recrystallization of lactide, comprising 
       (a) heating lactide to a temperature suffiecent to form a melt;  
       (b) cooling the melt in at least one recrystallizer to cause a lactide composition having a relatively higher content of a major isomer to crystallize on a surface of the recrystallizer while maintaining a lactide composition having a relatively lower content of the major isomer and a relatively higher content of impurities in a liquid phase; and  
       (c) separating the crystallized lactide composition from the liquid lactide composition.  
     
     
       12. The process of claim  11 , wherein the lactide in step (a) comprises a non-eutectic lactide mixture derived from the depolymerization of lactic acid oligomers comprising by weight a major amount of at least one lactide isomer and a minor amount of at least one other lactide isomer and acidic and other impurities. 
     
     
       13. The process of claim  11 , wherein said process comprises a solvent-free process. 
     
     
       14. The process of claim  11 , wherein steps (a) through (c) are repeated on the crystallized lactide composition obtained in step (c) until a crystallized lactide composition having a predetermined content of the major isomer is obtained. 
     
     
       15. The process of claim  11 , further comprising the steps of 
       (d) gradually warming the crystallized lactide composition to a temperature below its melting point to form a liquid phase; and  
       (e) separating the liquid phase from the crystallized lactide composition.  
     
     
       16. The process of claim  15 , wherein the lactide in step (a) comprises a non-eutectic lactide mixture derived from the depolymerization of lactic acid oligomers comprising by weight a major amount of at least one lactide isomer and a minor amount of at least one other lactide isomer and acidic and other impurities. 
     
     
       17. The process of claim  16 , wherein steps (a) through (e) are repeated on the crystallized lactide composition obtained in step (e) until a crystallized lactide composition having a predetermine copntent of the major isomer is obtained. 
     
     
       18. The process of claim  15 , wherein steps (a) through (e) are repeated on the crystallzed lactide composition obtained in step (e) until a crystallized lactide composition having a predetermined content of a major isomer is obtained. 
     
     
       19. The process of claim  11 , wherein the step (a) comprises heating lactide to a temperature of 110° C. and wherein step (b) comprises cooling the melt to a temperature of 90° C. 
     
     
       20. The method of claim  5  carried out in a falling film crystallizer. 
     
     
       21. The method of claim  5  carried out in a static crystallizer. 
     
     
       22. The method of claim  5  carried out in a zone melt purifier.